Metallic corrosion is a loss of material that occurs when metals are exposed to hostile environments. The chemical products of corrosion are oxidized products, where, by oxidized products are meant products containing the metal in a non-zero valence state.
The "return" of metals from a useful metallic state to an oxidized state imposes a massive cost on industrial economies. The annual economic burden of corrosion has been estimated to be on the order of 1 percent of the gross national products of the industrialized countries.
From thermodynamic considerations, the corrosion reaction (1), EQU M+M'.sup.+ =M.sup.+ +M' (1)
where M is the metal, and M'.sup.+ is another cation, can occur spontaneously if the equilbrium electrode potential for reaction (2), EQU M'.sup.+ +1e.sup.- =M' (2)
is greater than the equilibrium electrode potential for reaction (3), EQU M.sup.+ +1e.sup.-=M. ( 3)
This is because the oxidation reaction may be driven by coupling it to the reduction of the cation. Thus, the oxidation of iron, reaction (4), EQU 1/2Fe=1/2Fe.sup.++ +`E.sup.-, (4)
which has an equilibrium electrode potential for the reaction 4(a), EQU 1/2Fe.sup.++B +1 e.sup.- -1/2Fe 4(a)
of -0.44402 V, can be driven by coupling it to a reaction having a higher equilibrium electrode potential. In the case of the oxidation of iron, reaction (4), the driving reactions include: EQU H.sup.++ e.sup.- =1/2H.sub.2, (5) EQU H.sup.+ +1/4O.sub.2 +e.sup.- =1/2H.sub.2), (6) and EQU 1/2H.sub.2 O+1/4O.sub.2 +e.sup.- =OH.sup.- (7)
where reactions (5) and (6) are in acidic media, and reaction (7) is an alkaline media.
Various factors increase the rate of corrosion. These include the nature of the oxidizing reactants present, that is, the reactivity, concentration and temperature of the oxidizing environment; and the nature of the material, i.e. working history, thermal history, granularity, grain size, and grain orientation, among others. Additionally, kinetic factors such as bulk diffusion and electron transfer reactions determine the rate and progress of corrosion.
Corrosion may be in the form of uniform attack, characterized by progressive and uniform thinning of the metal and either the growth of an oxide or the loss of material. Alternatively, the corrosion may be a nonuniform corrosion, exemplified by galvanic corrosion arising from the juxtaposition of two or more metals. Galvanic attack is evidenced by dissolution of the more reactive metal.
Another form of a nonuniform corrosion is crevice corrosion which results in corrosion at flanges, the meeting of cross members, breaks in surface coatings, at a meniscus, and at water lines.
Various effects are observed in nonuniform corrosion. One effect is selective grain boundary attack which can cause whole crystallographic grains of the metal to fall out resulting in layer corrosion. Another effect of nonuniform corrosion is the preferential dissolution of one component in an alloy. Still another effect of non-uniform corrosion is the selective dissolution of a crystallographic grain along one crystallographic orientation. Other effects of nonuniform corrosion result from variations in reactivity brought about by surface films, surface oxides, nitriding and the like, such as pitting attack.
Additionally, mechanical stress may increase corrosion. For example mechanical stress results in stress corrosion cracking, which is a form of crevice attack at cracks that develop and constitute a self-perpetuating region of localized attack. Erosion corrosion, also known as impingement corrosion, occurs as the result of impingement of entrained particles in a stream of corrosive material. Mechanical effects of corrosion are evidenced by hydrogen embrittlement and corrosion fatique.
Various means have been attempted to limit corrosion. These include sacrificial anodes, impressed current cathodic protection, galvanization, formation of stable oxides, and protective coatings. Coating the surface, as with a paint, polymer, or metalloid, provides an impermeable layer which eliminates contact between the corrosive medium and the metal. However, in order to be effective, coatings must be adherent and pin-hole free. This is because defects allow a corrosive medium, e.g., water, chloride ion, sulfate ion, sulfur compounds, or oxygen, access to the metal. The corrosive medium can then supply electrons at the edges of the pin-hole. The resulting corrosion takes place under the coating, where the extent of damage is difficult to assess. Moreover, lack of adhesion can cause the coating to delaminate or develop holes or voids. One class of protective coatings found to be particularly desirable are coatings of disordered materials, for example coatings of iron with various oxide forming metals, and coatings of iron with various nonmetals deposited or formed under conditions that result in a disordered material. Corrosion can also be reduced by suitable alloying agents, as is the case with stainless steels.
Stainless steels are desired for their normally high corrosion resistance and attractive appearance. However, one problem encountered with stainless steels, especially when used for exterior applications, is atmospheric corrosion. Atmospheric corrosion is especially severe in marine atmospheres, where chloride ion is present in atomospheric water vapor, and in industrially polluted atmospheres, where various sulfur compounds are present in the atmosphere. Coatings that are capable of reducing the effects of atmospheric corrosion do so at the expense of the appearance of the stainless steel.